Short Gamma-ray Burst Afterglows Research Articles

Kilonova and Optical Afterglow from Binary Neutron Star Mergers. I. Luminosity Function and Color Evolution

In the first work of this series, we adopt a GW170817-like viewing-angle-dependent kilonova model and the standard afterglow model with a light-curve distribution based on the properties of cosmological short gamma-ray burst afterglows to simulate the luminosity functions and color evolution of both kilonovae and optical afterglow emissions from binary neutron star (BNS) mergers. We find that ∼10% of the nearly-on-axis afterglows are brighter than the associated kilonovae at the peak time. These kilonovae would be significantly polluted by the associated afterglow emission. Only at large viewing angles with , the electromagnetic signals of most BNS mergers would be kilonova-dominated and some off-axis afterglows may emerge at ∼5–10 days after the mergers. At a brightness dimmer than ∼23–24 mag, according to their luminosity functions, the number of afterglows is much larger than that of kilonovae. Because the search depth of the present survey projects is <22 mag, the number of afterglow events that are detected via serendipitous observations would be much higher than that of kilonova events, consistent with the current observations. For the foreseeable survey projects (e.g., Mephisto, WFST, and LSST), whose search depths can reach ≳23–24 mag, the detection rate of kilonovae could have the same order of magnitude as afterglows. We also find that it may be difficult to use the fading rate in a single band to directly identify kilonovae and afterglows among various fast-evolving transients by serendipitous surveys. However, the color evolution between the optical and infrared bands can identify them because the color evolution patterns of these phenomena are unique compared with those of other fast-evolving transients.

In Search of Short Gamma-Ray Burst Optical Counterparts with the Zwicky Transient Facility

The Fermi Gamma-ray Burst Monitor (GBM) triggers on-board in response to ∼40 short gamma-ray bursts (SGRBs) per year; however, their large localization regions have made the search for optical counterparts a challenging endeavour. We have developed and executed an extensive program with the wide field of view of the Zwicky Transient Facility (ZTF) camera, mounted on the Palomar 48 inch Oschin telescope (P48), to perform target-of-opportunity (ToO) observations on 10 Fermi-GBM SGRBs during 2018 and 2020–2021. Bridging the large sky areas with small field-of-view optical telescopes in order to track the evolution of potential candidates, we look for the elusive SGRB afterglows and kilonovae (KNe) associated with these high-energy events. No counterpart has yet been found, even though more than 10 ground-based telescopes, part of the Global Relay of Observatories Watching Transients Happen (GROWTH) network, have taken part in these efforts. The candidate selection procedure and the follow-up strategy have shown that ZTF is an efficient instrument for searching for poorly localized SGRBs, retrieving a reasonable number of candidates to follow up and showing promising capabilities as the community approaches the multi-messenger era. Based on the median limiting magnitude of ZTF, our searches would have been able to retrieve a GW170817-like event up to ∼200 Mpc and SGRB afterglows to z = 0.16 or 0.4, depending on the assumed underlying energy model. Future ToOs will expand the horizon to z = 0.2 and 0.7, respectively.

On the Use of CHIME to Detect Long-duration Radio Transients from Neutron Star Mergers

Abstract The short gamma-ray burst (SGRB) GRB 170817A was found to be related to a binary neutron star (BNS) merger. It is uncertain whether all SGRBs are caused by BNS mergers and also under what conditions a BNS merger can cause an SGRB. As BNS mergers can cause SGRBs, afterglow observations will also provide an alternative measurement of the BNS merger rate independent of gravitational-wave observations. In previous work by Feng et al., the feasibility of the detection of afterglows was considered using a variety of radio observatories and a simple flux threshold detection algorithm. Here, we consider a more sophisticated detection algorithm for SGRB afterglows and provide an estimate of the trials factors for a realistic search to obtain an updated estimate of the possibility of observing afterglows with the Canadian Hydrogen Intensity Mapping Experiment (CHIME). We estimate 893 and 312 afterglows per year can be detected using a 3σ confidence level threshold with two jet models, one with half-opening angle uniformly distributed in 6°–30° and the other uniformly distributed in 3°–8° with the median of 6°. We also find that 88% and 98%, respectively, of the detectable afterglows for each jet-opening distribution are off axis, which are candidates for orphan afterglows. Our result predicts fewer detectable sources per year than the earlier analysis but confirms the essential conclusion that using CHIME to search for afterglows will be effective in constraining the astrophysical merger rate.

The Gravity Collective: A Search for the Electromagnetic Counterpart to the Neutron Star–Black Hole Merger GW190814

Abstract We present optical follow-up imaging obtained with the Katzman Automatic Imaging Telescope, Las Cumbres Observatory Global Telescope Network, Nickel Telescope, Swope Telescope, and Thacher Telescope of the LIGO/Virgo gravitational wave (GW) signal from the neutron star–black hole (NSBH) merger GW190814. We searched the GW190814 localization region (19 deg2 for the 90th percentile best localization), covering a total of 51 deg2 and 94.6% of the two-dimensional localization region. Analyzing the properties of 189 transients that we consider as candidate counterparts to the NSBH merger, including their localizations, discovery times from merger, optical spectra, likely host galaxy redshifts, and photometric evolution, we conclude that none of these objects are likely to be associated with GW190814. Based on this finding, we consider the likely optical properties of an electromagnetic counterpart to GW190814, including possible kilonovae and short gamma-ray burst afterglows. Using the joint limits from our follow-up imaging, we conclude that a counterpart with an r-band decline rate of 0.68 mag day−1, similar to the kilonova AT 2017gfo, could peak at an absolute magnitude of at most −17.8 mag (50% confidence). Our data are not constraining for “red” kilonovae and rule out “blue” kilonovae with M &gt; 0.5 M ⊙ (30% confidence). We strongly rule out all known types of short gamma-ray burst afterglows with viewing angles &lt;17° assuming an initial jet opening angle of ∼5.°2 and explosion energies and circumburst densities similar to afterglows explored in the literature. Finally, we explore the possibility that GW190814 merged in the disk of an active galactic nucleus, of which we find four in the localization region, but we do not find any candidate counterparts among these sources.

Kilonova Emission from Black Hole–Neutron Star Mergers. II. Luminosity Function and Implications for Target-of-opportunity Observations of Gravitational-wave Triggers and Blind Searches

Abstract We present detailed simulations of the kilonova and gamma-ray burst (GRB) afterglow and kilonova luminosity function from black hole–neutron star (BH–NS) mergers, and discuss the detectability of an electromagnetic (EM) counterpart in connection with gravitational wave (GW) detections, GW-triggered target-of-opportunity observations, and time-domain blind searches. The predicted absolute magnitude of BH–NS kilonovae at 0.5 days after the merger falls in the range [−10, −15.5]. The simulated luminosity function contains potential information on the viewing-angle distribution of the anisotropic kilonova emission. We simulate the GW detection rates, detectable distances, and signal duration for future networks of 2nd/2.5th/3rd generation GW detectors. BH–NSs tend to produce brighter kilonovae and afterglows if the BH has a higher aligned spin, and a less massive NS with a stiffer equation of state. The detectability of kilonovae is especially sensitive to the BH spin. If BHs typically have low spins, the BH–NS EM counterparts are hard to discover. For 2nd generation GW detector networks, a limiting magnitude of m limit ∼ 23–24 mag is required to detect kilonovae even if high BH spin is assumed. Thus, a plausible explanation for the lack of BH–NS-associated kilonova detection during LIGO/Virgo O3 is that either there is no EM counterpart (plunging events) or the current follow-ups are too shallow. These observations still have the chance to detect the on-axis jet afterglow associated with a short GRB or an orphan afterglow. Follow-up observations can detect possible associated short GRB afterglows, from which kilonova signatures may be studied. For time-domain observations, a high-cadence search in redder filters is recommended to detect more BH–NS-associated kilonovae and afterglows.

DDOTI observations of gravitational-wave sources discovered in O3

ABSTRACT We present optical follow-up observations with the Deca-Degree Optical Transient Imager (DDOTI) telescope of gravitational-wave (GW) events detected during the Advanced LIGO and Advanced Virgo O3 observing run. DDOTI is capable of responding to an alert in a few minutes, has an instantaneous field of about 69 deg2, and obtains 10σ upper limits of wlim = 18.5–20.5 AB mag in 1000 s of exposure, depending on the conditions. We observed 54 per cent (26 out of 48) of the unretracted GW alerts and did not find any electromagnetic counterparts. We compare our upper limits to various possible counterparts: the kilonova AT 2017gfo, models of radioactive- and magnetar-powered kilonovae, short gamma-ray burst afterglows, and active galactic nucleus (AGN) flares. Although the large positional uncertainties of GW sources do not allow us to place strong constraints during O3, DDOTI observations of well-localized GW events in O4 and beyond could meaningfully constrain models of compact binary mergers. We show that DDOTI is able to detect kilonovae similar to AT 2017gfo up to about 200 Mpc and magnetar-powered kilonovae up to 1 Gpc. We calculate that nearby (≲200 Mpc) afterglows have a high chance (≈70 per cent) to be detected by rapid (≲3 h) DDOTI observations if observed on-axis, whereas off-axis afterglows are unlikely to be seen. Finally, we suggest that long-term monitoring of massive BBH events with DDOTI could confirm or rule out late AGN flares associated with these events.

Detectability of radio afterglows from binary neutron star mergers and implications for fast radio bursts

ABSTRACT Binary neutron star (BNS) mergers are one of the proposed origins for both repeating and non-repeating fast radio bursts (FRBs), which associates FRBs with gravitational waves and short gamma-ray bursts (GRBs). In this work, we explore detectability of radio afterglows from BNS mergers and compare it to the observed radio limits on FRB afterglow. We calculate the afterglow flux powered by the two components: a relativistic jet and a slower isotropic ejecta, and quantify the detection probability as a function of the source redshift, observing time, and flux sensitivity. The model parameter distributions inferred from short GRB afterglows are adopted, and viewing angle distributions (uniform spherical, gravitational-wave, on-axis biased) are assumed to reflect different searching scenario. Assuming that FRBs are not strongly beamed, we make comparison to FRBs detected with reported radio limits and find the detection probabilities are 1–10 per cent in general, and hence not a strong constraint on the BNS progenitor model considering the small sample number (&amp;lt;10). In particular for some nearby FRBs (e.g. 180916.J0158+65, 190608), we find a high chance of detection (&amp;gt;20 per cent at 10 μJy sensitivity) for the isotropic component that would peak around ∼1–10 yr after the merger. Therefore, a long-term radio monitoring of persistent radio emission for these objects is important.

Fast Extragalactic X-Ray Transients from Gamma-Ray Bursts Viewed Far Off-axis

Abstract The observed light curves and estimated sky rate of fast extragalactic X-ray transients (XRTs) discovered in archival Chandra data indicate that they belong to two distinct XRT populations. The first population of relatively short duration pulses, which typically last less than few minutes seems to be pulses of X-ray flashes, which are nearby long-duration gamma-ray bursts viewed from far off-axis. The second population of much longer pulses, which typically last hours, seems to be the early-time isotropic afterglows of short gamma-ray bursts that are beamed away from Earth, as was shown in a previous paper.

On-axis view of GRB 170817A

The peculiar short gamma-ray burst (SGRB) GRB 170817A has been firmly associated to the gravitational wave event GW170817, which has been unanimously interpreted as due to the coalescence of a double neutron star binary. The unprecedented behaviour of the non-thermal afterglow led to a debate over its nature, which was eventually settled by high-resolution VLBI observations that strongly support the off-axis structured jet scenario. Using information on the jet structure derived from multi-wavelength fitting of the afterglow emission and of the apparent VLBI image centroid motion, we compute the appearance of a GRB 170817A-like jet as seen by an on-axis observer and compare it to the previously observed population of SGRB afterglows and prompt emission events. We find that the intrinsic properties of the GRB 170817A jet are representative of a typical event in the SGRB population, hinting at a quasi-universal jet structure. The diversity in the SGRB afterglow population could therefore be ascribed in large part to extrinsic (redshift, density of the surrounding medium, viewing angle) rather than intrinsic properties. Although more uncertain, the comparison can be extended to the prompt emission properties, leading to similar conclusions.

Follow-up of the Neutron Star Bearing Gravitational-wave Candidate Events S190425z and S190426c with MMT and SOAR

Abstract On 2019 April 25.346 and 26.640 UT the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational-wave (GW) observatory announced the detection of the first candidate events in Observing Run 3 that contained at least one neutron star (NS). S190425z is a likely binary neutron star (BNS) merger at d L = 156 ± 41 Mpc, while S190426c is possibly the first NS–black hole (BH) merger ever detected, at d L = 377 ± 100 Mpc, although with marginal statistical significance. Here we report our optical follow-up observations for both events using the MMT 6.5 m telescope, as well as our spectroscopic follow-up of candidate counterparts (which turned out to be unrelated) with the 4.1 m SOAR telescope. We compare to publicly reported searches, explore the overall areal coverage and depth, and evaluate those in relation to the optical/near-infrared (NIR) kilonova emission from the BNS merger GW170817, to theoretical kilonova models, and to short gamma-ray burst (SGRB) afterglows. We find that for a GW170817-like kilonova, the partial volume covered spans up to about 40% for S190425z and 60% for S190426c. For an on-axis jet typical of SGRBs, the search effective volume is larger, but such a configuration is expected in at most a few percent of mergers. We further find that wide-field γ-ray and X-ray limits rule out luminous on-axis SGRBs, for a large fraction of the localization regions, although these searches are not sufficiently deep in the context of the γ-ray emission from GW170817 or off-axis SGRB afterglows. The results indicate that some optical follow-up searches are sufficiently deep for counterpart identification to about 300 Mpc, but that localizations better than 1000 deg2 are likely essential.

Systematic study for gas-to-dust ratio of short gamma-ray burst afterglows

Abstract Extragalactic X-ray absorption and optical extinction are often found in gamma-ray burst (GRB) afterglows, and they could be tracers of both circumburst and host galaxy environments. By performing spectral analyses of the spectral energy distribution of nine short GRB (SGRB) afterglows with a known redshift, we investigated the ratio of the equivalent hydrogen column density to the dust extinction, $N_{\rm H}^{\rm rest}/A_{V}^{\rm rest}$, in the rest frame of each SGRB. We found that the distribution of $N_{\rm H}^{\rm rest}/A_{V}^{\rm rest}$ is systematically smaller than for long GRBs, and is roughly consistent with the gas-to-dust ratio in the Milky Way. This result means that the measured gas-to-dust ratio of SGRBs would originate from the interstellar medium in each host galaxy. This scenario supports the prediction that SGRBs occur in non-star-forming regions in the host galaxies.

A luminosity distribution for kilonovae based on short gamma-ray burst afterglows

Abstract The combined detection of a gravitational-wave signal, kilonova, and short gamma-ray burst (sGRB) from GW170817 marked a scientific breakthrough in the field of multimessenger astronomy. But even before GW170817, there have been a number of sGRBs with possible associated kilonova detections. In this work, we re-examine these ‘historical’ sGRB afterglows with a combination of state-of-the-art afterglow and kilonova models. This allows us to include optical/near-infrared synchrotron emission produced by the sGRB as well as ultraviolet/optical/near-infrared emission powered by the radioactive decay of r-process elements (i.e. the kilonova). Fitting the light curves, we derive the velocity and the mass distribution as well as the composition of the ejected material. The posteriors on kilonova parameters obtained from the fit were turned into distributions for the peak magnitude of the kilonova emission in different bands and the time at which this peak occurs. From the sGRB with an associated kilonova, we found that the peak magnitude in H bands falls in the range [−16.2, −13.1] ($95{{\ \rm per\ cent}}$ of confidence) and occurs within $0.8\!-\!3.6\, \rm d$ after the sGRB prompt emission. In g band instead we obtain a peak magnitude in range [−16.8, −12.3] occurring within the first 18 h after the sGRB prompt. From the luminosity distributions of GW170817/AT2017gfo, kilonova candidates GRB130603B, GRB050709, and GRB060614 (with the possible inclusion of GRB150101B, GRB050724A, GRB061201, GRB080905A, GRB150424A, and GRB160821B) and the upper limits from all the other sGRBs not associated with any kilonova detection we obtain for the first time a kilonova luminosity distribution in different bands.

Detecting the imprint of a kilonova or supernova in short gamma-ray burst afterglows

Context. Short gamma-ray bursts (GRBs) result from mergers of two neutron stars or from collapsars, but probably at a smaller rate. In the first case, a kilonova occurs while in the second case a Type Ic supernova is expected. Aims. Even if future observations of kilonovae in association with gravitational wave events provide better data, detecting a kilonova during an afterglow follow-up would remain useful for exploring the diversity of the kilonova phenomenon. As supernovae produce a weaker gravitational signal, afterglow follow-up will be the only possible method to find one. In this work, we identify the conditions of the burst energy, external density, kilonova mass, supernova luminosity, that are necessary for the detection of a kilonova or supernova in the follow-up of short GRB afterglows. Methods. We have used a simple kilonova model to obtain the peak luminosities and times as a function of mass, expansion velocity and ejected matter opacity. Afterglow light curves are computed for a uniform medium and a stellar wind, in the kilonova and supernova cases, respectively. Results. We represent, using diagrams of the burst kinetic energy vs. density of the external medium, the domains where the kilonova or supernova at maximum is brighter than the afterglow. In the kilonova case we vary the mass, the jet opening angle and the microphysics parameters; for supernovae, we consider SN 98bw-like and ten times dimmer events, and again vary the jet opening angle and the microphysics parameters.

Brightening X-Ray/Optical/Radio Emission of GW170817/SGRB 170817A: Evidence for an Electron–Positron Wind from the Central Engine?

Abstract Recent follow-up observations of the binary neutron star (NS) merging event GW170817/SGRB 170817A reveal that its X-ray/optical/radio emissions are brightening continuously up to ∼100 days post-merger. This late-time brightening is unexpected from the kilonova model or the off-axis top-hat jet model for gamma-ray burst (SGRB) afterglows. In this Letter, by assuming that the merger remnant is a long-lived NS, we propose that the interaction between an electron–positron-pair (e + e −) wind from the central NS and the jet could produce a long-lived reverse shock, from which a new emission component would rise and can interpret current observations well. The magnetic-field-induced ellipticity of the NS is taken to be 4 × 10−5 in our modeling, so that the braking of the NS is mainly through the gravitational wave (GW) radiation rather than the magnetic dipole radiation, and the emission luminosity at early times would not exceed the observational limits. In our scenario, because the peak time of the brightening is roughly equal to the spin-down timescale of the NS, the accurate peak time may help constrain the ellipticity of the remnant NS. We suggest that radio polarization observations of the brightening would help to distinguish our scenario from other scenarios. Future observations on a large sample of short gamma-ray burst afterglows or detections of GW signals from merger remnants would test our scenario.

The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/Virgo GW170817. VIII. A Comparison to Cosmological Short-duration Gamma-Ray Bursts

Abstract We present a comprehensive comparison of the properties of the radio through X-ray counterpart of GW170817 and the properties of short-duration gamma-ray bursts (GRBs). For this effort, we utilize a sample of 36 short GRBs spanning a redshift range of z ≈ 0.12 – 2.6 discovered over 2004–2017. We find that the counterpart to GW170817 has an isotropic-equivalent luminosity that is ≈ 3000 times less than the median value of on-axis short GRB X-ray afterglows, and ≳104 times less than that for detected short GRB radio afterglows. Moreover, the allowed jet energies and particle densities inferred from the radio and X-ray counterparts to GW170817 and on-axis short GRB afterglows are remarkably similar, suggesting that viewing angle effects are the dominant, and perhaps only, difference in their observed radio and X-ray behavior. From comparison to previous claimed kilonovae following short GRBs, we find that the optical and near-infrared (NIR) counterpart to GW170817 is comparatively under-luminous by a factor of ≈ 3 – 5 , indicating a range of kilonova luminosities and timescales. A comparison of the optical limits following short GRBs on ≲ 1 day timescales also rules out a “blue” kilonova of comparable optical isotropic-equivalent luminosity in one previous short GRB. Finally, we investigate the host galaxy of GW170817, NGC 4993, in the context of short GRB host galaxy stellar population properties. We find that NGC 4993 is superlative in terms of its large luminosity, old stellar population age, and low star formation rate compared to previous short GRB hosts. Additional events within the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO)/Virgo volume will be crucial in delineating the properties of the host galaxies of neutron star–neutron star (NS–NS) mergers, and connecting them to their cosmological counterparts.

Magnetorotational instability in neutron star mergers: impact of neutrinos

The merger of two neutron stars may give birth to a long-lived hypermassive neutron star. If it harbours a strong magnetic field of magnetar strength, its spin-down could explain several features of short gamma-ray burst afterglows. The magnetorotational instability (MRI) has been proposed as a mechanism to amplify the magnetic field to the required strength. Previous studies have, however, neglected neutrinos, which may have an important impact on the MRI by inducing a viscosity and drag. We investigate the impact of these neutrino effects on the linear growth of the MRI by applying a local stability analysis to snapshots of a neutron star merger simulation. We find that neutrinos have a significant impact inside the hypermassive neutron star, but have at most a marginal effect in the torus surrounding it. Inside the hypermassive neutron star, the MRI grows in different regimes depending on the radius and on the initial magnetic field strength. For magnetic fields weaker than $10^{13}-10^{14}\,{\rm G}$, the growth rate of the MRI is significantly reduced due to the presence of neutrinos. We conclude that neutrinos should be taken into account when studying the growth of the MRI from realistic initial magnetic fields. Current numerical simulations, which neglect neutrino viscosity, are only consistent, i.e. in the adopted ideal regime, if they start from artificially strong initial magnetic fields above $\sim10^{14}\,{\rm G}$. One should be careful when extrapolating these results to lower initial magnetic fields, where the MRI growth is strongly affected by neutrino viscosity or drag.

RADIO CONSTRAINTS ON LONG-LIVED MAGNETAR REMNANTS IN SHORT GAMMA-RAY BURSTS

ABSTRACT The merger of a neutron star (NS) binary may result in the formation of a rapidly spinning magnetar. The magnetar can potentially survive for seconds or longer as a supramassive NS before collapsing to a black hole if, indeed, it collapses at all. During this process, a fraction of the magnetar’s rotational energy of ∼1053 erg is transferred via magnetic spin-down to the surrounding ejecta. The resulting interaction between the ejecta and the surrounding circumburst medium powers a year-long or greater synchrotron radio transient. We present a search for radio emission with the Very Large Array following nine short-duration gamma-ray bursts (GRBs) at rest-frame times of ≈1.3–7.6 yr after the bursts, focusing on those events that exhibit early-time excess X-ray emission that may signify the presence of magnetars. We place upper limits of ≲18–32 μJy on the 6.0 GHz radio emission, corresponding to spectral luminosities of ≲(0.05–8.3) × 1039 erg s−1. Comparing these limits to the predicted radio emission from a long-lived remnant and incorporating measurements of the circumburst densities from broadband modeling of short GRB afterglows, we rule out a stable magnetar with an energy of 1053 erg for half of the events in our sample. A supramassive remnant that injects a lower rotational energy of 1052 erg is ruled out for a single event, GRB 050724A. This study represents the deepest and most extensive search for long-term radio emission following short GRBs to date, and thus the most stringent limits placed on the physical properties of magnetars associated with short GRBs from radio observations.

JET COLLIMATION IN THE EJECTA OF DOUBLE NEUTRON STAR MERGERS: A NEW CANONICAL PICTURE OF SHORT GAMMA-RAY BURSTS

The observations of jet breaks in the afterglows of short gamma-ray bursts (SGRBs) indicate that the jet has a small opening angle of < 10{\deg}. The collimation mechanism of the jet is a longstanding theoretical problem. We numerically analyze the jet propagation in the material ejected by double neutron star merger, and demonstrate that if the ejecta mass is > 10^{-2} M_{sun}, the jet is well confined by the cocoon and emerges from the ejecta with the required collimation angle. Our results also suggest that there are some populations of choked (failed) SGRBs or low-luminous new types of event. By constructing a model for SGRB 130603B, which is associated with the first kilonova/macronova can- didate, we infer that the equation-of-state of neutron stars would be soft enough to provide sufficient ejecta to collimate the jet, if this event was associated with a double neutron star merger.

Short-Duration Gamma-Ray Bursts

Gamma-ray bursts (GRBs) display a bimodal duration distribution with a separation between the short- and long-duration bursts at about 2 s. The progenitors of long GRBs have been identified as massive stars based on their association with Type Ic core-collapse supernovae (SNe), their exclusive location in star-forming galaxies, and their strong correlation with bright UV regions within their host galaxies. Short GRBs have long been suspected on theoretical grounds to arise from compact object binary mergers (neutron star–neutron star or neutron star–black hole). The discovery of short GRB afterglows in 2005 provided the first insight into their energy scale and environments, as well as established a cosmological origin, a mix of host-galaxy types, and an absence of associated SNe. In this review, I summarize nearly a decade of short GRB afterglow and host-galaxy observations and use this information to shed light on the nature and properties of their progenitors, the energy scale and collimation of the relativistic outflow, and the properties of the circumburst environments. The preponderance of the evidence points to compact object binary progenitors, although some open questions remain. On the basis of this association, observations of short GRBs and their afterglows can shed light on the on- and off-axis electromagnetic counterparts of gravitational wave sources from the Advanced LIGO/Virgo experiments.

Signature of gravitational wave radiation in afterglows of short gamma-ray bursts?

Short gamma-ray bursts (GRBs), brief intense emission of $\ensuremath{\gamma}$ rays characterized by a duration shorter than 2 s that are plausibly powered by the coalescence of binary neutron stars, are believed to be strong gravitational wave radiation (GWR) sources. The test of such a speculation has been thought to be impossible until the performance of the detectors like advanced LIGO. Recently there has been growing evidence for the formation of a highly magnetized neutron star (i.e., magnetar) in the double neutron star mergers. In this work we reexamine the interpretation of the x-ray plateau followed by an abrupt decline detected in some short GRB afterglows within the supramassive magnetar model and find that the maximum gravitational mass of the nonrotating neutron stars is $\ensuremath{\sim}2.3{M}_{\ensuremath{\bigodot}}$ and the observed duration of some x-ray plateaus are significantly shorter than that expected in the magnetic dipole radiation scenario, suggesting that the collapse of the supramassive magnetars has been considerably enhanced by the energy loss via GWR. Such a result demonstrates that the signature of GWR may have already existed in current electromagnetic data of short GRBs.